When conducting polymerizations on a commercial basis, it is important to utilize process conditions and components which will allow tile molecular weight of the end products to be narrowly and reproducibly defined. The characteristics of a given polymer and its usefulness, are dependent, among other things, upon its molecular weight. Hence, it is desirable to be able to predict with some certainty the molecular weight of the end product of the polymerization. When the molecular weight is not narrowly definable, or is ,lot reproducible on a systematic basis, the process is not commercially viable.
In the art, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis characteristics. Such elastomers, when compounded to form articles such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance and will have less heat build-up when mechanical stresses are applied.
A major source of hysteretic power loss has been established to be due to the section of the polymer chain from the last cross link of the vulcanizate to an end of the polymer chain. This free end cannot tie involved in an efficient elastically recoverable process, and as a result, any energy transmitted to this section of the cured sample is lost as heat. It is known in the art that this type of mechanism can be reduced by preparing higher molecular weight polymers which will have fewer end groups. However, this procedure is not useful because processability of the rubber with compounding ingredients and during shaping operations decreases rapidly with increasing molecular weight.
It is difficult to obtain consistent properties, such as a reduction in hysteresis properties, if the polymer cannot be controllably reproduced in a narrow molecular weight range distribution. See, for example, U.S. Pat. No. 4,935,471, in which some polymers are prepared with a heterogeneous mixture of certain secondary amines, including lithium pyrrolidide. Polymers made in this manner have widely variable molecular weights, broad polydispersities, their functional terminations tend to reproduce erratically, giving rise to poorly reproducible hysteresis reduction results.
It is known in the art to employ a lithium amide with an alkali metal compound, such as, for example, as discussed in Japanese Pat. No. 7,965,788. That patent does not disclose the use of a chelating agent as in tile present invention. Polymerization to form certain rubber compounds such as styrene/butadiene rubber (SBR) with higher styrene content by using only a lithium amide initiator and alkali metal compound randomizers in acyclic alkanes, causes tile formation of a heterogeneous polymer cement, which is comprised of a mixture of widely different styrene content molecules. This makes it difficult to achieve the desired molecular weight and to difficult to control styrene sequence distribution. The presence of such cements often interferes with desirable rubber properties in the polymer.
Furthermore, a major drawback with many of these known initiators, is that they are not soluble in hydrocarbon solvents such as hexane or cyclohexene. Polar solvents have heretofore been employed including the polar organic ethers such as dimethyl or diethyl ether, tetrahydrofuran, tetramethylelhylenediamine, or diethylene glycol methyl ether (diglyme).
The invention also provides for the incorporation of a functionality from the initiator to be incorporated into the polymer chain, such that two or more of tile ends of the resulting polymer chains are modified. Hysteresis characteristics of the resulting products are effectively reduced, and other physical characteristics are improved. The invention provides for efficient, controllable and reproducible polymerizations, with the preparation of well defined end-products of a relatively narrow molecular weight distribution range. Furthermore, there is provided a means of controlling the sequence distribution of vinyl aromatic monomers, such as styrene, along a polymer backbone, to improve the hysteresis properties, tear strength and wear resistance of tile resulting products.